This invention relates to a reference voltage producing circuit fabricated in an integrated circuit and, more particularly, to a reference voltage producing circuit fabricated in a bipolar IC.
A known circuit, called a band-gap reference circuit, has been used for the reference voltage producing circuit in fabrication of the bipolar IC. FIG. 1 shows a circuit diagram for illustrating the principles of the band-gap reference circuit. In FIG. 1, the circuit includes an NPN transistor Q.sub.1 in which the collector-emitter path is connected between the reference voltage output terminals .sym. and .crclbar. through resistors R.sub.1 and R.sub.2 and the base electrode is connected to the collector, and an NPN transistor Q.sub.2 in which the emitter-collector path is connected between the reference voltage output terminals .sym. and .crclbar. via a resistor R.sub.3 and the base electrode is connected to the collector. An operational amplifier 1 is connected at the inverting input terminal (-) to a node a between the resistors R.sub.1 and R.sub.2, at the noninverting input terminal (+) to a node b between the resistor R.sub.3 and the collector of the transistor Q.sub.2, and at the output terminal to the reference voltage output 2erminal (+) and to a common junction between the resistors R.sub.1 and R.sub.3.
In FIG. 1, the operational amplifier 1 operates so that the potential levels at nodes a and b are equal to each other. If the resistances of the resistors R.sub.1 and R.sub.3 are set to be equal to each other and the emitter area of the transistor Q.sub.1 is set to be larger than that of the transistor Q.sub.2, the base-emitter voltage V.sub.BE1 of the transistor Q.sub.1 becomes smaller than the base-emitter voltage V.sub.BE1 of the transistor Q.sub.2 and a difference voltage of "V.sub.BE2 -V.sub.BE1 " appears across the resistor R.sub.2. More specifically, if V.sub.BE2 is 0.7 V, the base-emitter voltage V.sub.BE1 of the transistor Q.sub.1 is smaller than 0.7 volts and 0.7 volts is applied to the non-inverting input terminal (+) of the operation amplifier 1. If a resistance ratio of the resistance of the resistor R.sub.1 to that of the resistor R.sub.2 is so selected that the voltage drop across the resistor R.sub.1 is about 0.7 volts, a reference or output voltage V.sub.OUT of about 1.2 volts appears between the reference voltage output terminals .sym. and .crclbar., since the voltage levels at the input terminals (+) and (-) of the operational amplifier 1 are equal to each other.
The circuit of FIG. 1 provides a reference voltage or an output voltage V.sub.OUT with a small temperature coefficient, but has the following defects. In the operational amplifier 1, the switching operation is performed at a high speed, so that the reference voltage V.sub.OUT has a pulsative wave form which includes an AC component. Therefore, it is necessary to provide a capacitor for phase compensation in the operational amplifier in order to prevent the operational amplifier from oscillating due to this AC component. The capacitance of this phase compensation capacitor is small, 30 pF or so. However, this capacitor creates a problem when this capacitor is fabricated into an integrated circuit, because it needs a large area on the chip. That is, this capacitor hinders the improvement of integration density.